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In article ,
(DoN. Nichols) wrote:

According to Joseph Gwinn :
In article ,
(DoN. Nichols) wrote:

[ ... ]

Such a meter could indicate the voltage across the diode string, which
is logarithmic in current.

That could indeed be what they are for. The zero adjust pin was
broken off the zero adjust screw, so I guess that was intentional,
rather than someone being ham-fisted in putting the meter face back on.

If you can see the pole faces, does the magnetic gap vary with movement
angle? This is the other way to implement a log meter.

Nope! constant gap, as in most other meters that I have seen.

That pretty well clinches it.

Diodes are far cheaper than special meter movements.

I just spent this evening in the shop, making a fixture to mount
the meter movement onto while I'm working on it, since there is not
otherwise any reasonable way to grip it.

The fixture also has one of a bunch of plastic TV-dinner pans
(for something like macaroni and cheese) firmly attached between the
brass hex standoffs (brass to avoid interaction with the magnet). That
serves two functions, here.

1) To catch small parts if I drop them.

Unless they bounce right out. A pan liner of some kind may be necessary.

2) To keep metal filings from the workbench from jumping up to the
magnet (which can jam the meter movement -- I've seen it
before).

Good point. It's real hard to clean a magnet assembly that has
collected iron filings.

I'd also sweep the bench with a strong magnet in a bag first, to reduce
the supply of iron filings in the first place.

The work will be done under a stereo-zoom microscope (3-70X)
with its own illumination, and I'll be using non-magnetic tweezers for
manipulating the hair-spring.

The soldering iron (An old "Loner") is mostly a non-magnetic
alloy of stainless steel, so it should not fall victim to the magnet.
(But I will have to remove the entire movement from the magnet assembly
to gain access to the point which I need to re-solder.

A new retirement business - fixing meter movements. Then, move up to
crashed disk drives.

If this fails (for whatever reason), I've got a brother to the
system bookmarked on eBay. That one is missing quite a few parts,
including some special dials, which should keep the serious bids down.
It *does* have the meter, at least.

[ ... ]

Yep! That was the intention when our "customers" asked for that
feature [the pulse-forming network].

I recall a passing note in Edgerton's book saying that lots of people
had tried to use pulse-forming networks to drive flashlamps, but the
wild variations in flashlamp impedance during the flash undermined all
their efforts.

Probably why this did nothing useful. :-)

Yep. It worked pretty well with hard-vacuum tubes, like magnetrons.

O.K. That may be where they got the idea.

Exactly. It was mostly the same people working on both, at least the
MIT crowd. Edgerton was a MIT professor.

Ten 20-uF caps at 1000 Volts is 100 Joules. Ouch! Good it was not
from arm to arm. A defibrillator is 400 Joules.

But it *was* from arm to arm. Remember -- between left (index) finger
and right elbow.

It seemed to take five minutes before I realized that the noise
which was getting in the way of my checking for my heartbeat *was* my
heartbeat.

Shortly after that, I pushed for our lab to send everyone who
wanted it to a CPR class. These days, we would have to stock anti-AIDS
mouthpieces for use just in case. :-)

Right. You were very lucky in the timing (with respect to your
heartbeat) of the shock.

Agreed. FWIW -- I was quite young then, and I think that the
odds of my surviving such another time would be rather slim.

Actually, age has little to do with it. Timing is everything.

I also once (intentionally) dumped that cap bank into a 10 ohm 2
Watt carbon resistor. Parts of that wound up embedded in the ceiling
acoustic tile. :-) Yes -- I did make sure that nobody was in the path of
the expected shrapnel. :-)

Did you explode any wire?

Nope -- but I did (on a bet) cut a power cord with a pair of
diagonal pliers without insulating gloves. I just made sure to specify
that it would be *his* diagonal pliers, not mine. :-) The floor was
linoleum tile over concrete, and my shoes were rubber soled, so I was
well enough insulated, and as expected, the primary path of the
electrons was between hot and neutral through the expansing gasses of
what had once been the blades of the dikes. :-) Of course, the circuit
breaker tripped rather quickly.

The classic dodge is to nip the power cord halfway, cutting one wire at
a time. Easy with flat cable, harder with round cable, but not
impossible. But I assume that the point of the bet was to make a big
bang and yet survive to collect.

Joe Gwinn